export class Thread { id: string; lastMessage: Message; name: string; avatarSrc: string; constructor(id?: string, name?: string, avatarSrc?: string) { this.id = id || uuid(); this.name = name; this.avatarSrc = avatarSrc; }
}In id? what's the ? for?
4 Answers
It is to mark the parameter as optional.
- TypeScript handbook
- TypeScript Deep Dive
This is to make the variable of Optional type. Otherwise declared variables shows "undefined" if this variable is not used.
export interface ISearchResult { title: string; listTitle:string; entityName?: string, lookupName?:string, lookupId?:string
}parameter?: type is a shorthand for parameter: type | undefined
So what is the difference? Question mark means "optional".
More precisely parameter?: type is equal parameter: type | undefined = undefined
The ? in the parameters is to denote an optional parameter. The Typescript compiler does not require this parameter to be filled in. See the code example below for more details:
// baz: number | undefined means: the second argument baz can be a number or undefined
// = undefined, is default parameter syntax,
// if the parameter is not filled in it will default to undefined
// Although default JS behaviour is to set every non filled in argument to undefined
// we need this default argument so that the typescript compiler
// doesn't require the second argument to be filled in
function fn1 (bar: string, baz: number | undefined = undefined) { // do stuff
}
// All the above code can be simplified using the ? operator after the parameter
// In other words fn1 and fn2 are equivalent in behaviour
function fn2 (bar: string, baz?: number) { // do stuff
}
fn2('foo', 3); // works
fn2('foo'); // works
fn2();
// Compile time error: Expected 1-2 arguments, but got 0
// An argument for 'bar' was not provided.
fn1('foo', 3); // works
fn1('foo'); // works
fn1();
// Compile time error: Expected 1-2 arguments, but got 0
// An argument for 'bar' was not provided.
